The invention relates to an optical waveguide doped with a lanthanide ion-sensitizer complex. Optical waveguides doped with lanthanide ions are known from EP-A2-0,437,935, which discloses an optical waveguide composed of an Er3+ doped fiber. The erbium ions are excited with the aid of a laser, giving a fiber containing a large number of Er3+ ions excited to such an extent that population inversion occurs, i.e. that more Er3+ ions are in the excited state than in the ground state. When optical beams (photons) having the same wavelength as the emission wavelength of the excited Er3+ ions traverse the fiber, they effect the transition of the ions from the excited state to a lower energy level with transmission of light. This light has the same wavelength and phase as the photons traversing the fiber. Such a process is called stimulated emission. In this way the optical fiber acts as waveguide of the light beams.
The optical waveguide, as specified in EP-A2-0,437,935, is a glass fiber. For several years efforts have been made to replace optical glass fibers with optical polymeric fibers. Optical polymeric fibers have several advantages over optical glass fibers. They can be made by less complicated spinning processes, easily cut to the required size, and attached to receiving and transmitting devices. Also, they are lighter and more flexible than glass fibers. A further advantage is that the shape of the optical polymeric material is not restricted to fibers. The polymeric material can also be shaped into so-called planar waveguides. In the remainder of this description the term xe2x80x9coptical waveguidexe2x80x9d refers to both fibers and planar waveguides.
So far, it has not proved possible to replace lanthanide-doped optical waveguides with optical waveguides of polymeric material. This is because polymeric waveguides cannot be doped with lanthanide metals. In particular, lanthanide ions used for amplification in the telecommunication transmission windows in the near-IR are very sensitive to radiationless deactivation, which counteracts their sensitivity as optical waveguides. The deactivation is brought about by high-energy vibrations near the lanthanide ion. Most effective are Oxe2x80x94H, Nxe2x80x94H, and Cxe2x80x94H vibrational modes. The risk of co-doping with water is a major problem of organic lanthanide complexes, because trivalent lanthanide ions are extremely hygroscopic. In consequence, when lanthanide ions are doped, crystallization water which is present in the lanthanide salt is also introduced into the waveguide. Alternatively, lanthanide ions already present in the waveguide may interact with water or other OH-containing impurities. Water and other OH-containing impurities quench the excited state of the trivalent lanthanide ions. So, unless further steps are taken, a lanthanide-doped optical polymeric waveguide will not show the above-indicated amplification, or not show it in sufficient degree. Furthermore, light is absorbed by OH-groups to such an extent that polymeric optical waveguides, in which OH-impurities are present, will display optical attenuation. Doping of Er3+ ions in glass fibers ordinarily is carried out using Er2O3. This compound, however, cannot be used in polymeric optical waveguides since it fails to dissolve in polymeric material. When glass fibers contain water or OH-containing impurities, these can easily be removed by heating the fibers and drying them out. However, this is an unfortunate solution to the problem where polymeric optical waveguides are concerned, as they will usually decompose under such treatment.
The present invention has for its object to obviate these drawbacks and provide a functional lanthanide-doped optical waveguide in which a polymeric waveguide is used as optical material. Hence, the invention consists in that the polymeric waveguide comprises a lanthanide ion-sensitizer complex, wherein the sensitizer absorbs in the 400-1200 nm region, and preferably in the 600-1000 nm region.
The term xe2x80x9ccomplexxe2x80x9d in this connection refers to a compound in which the lanthanide ion is encapsulated by a host molecule. If a complex is provided in which trivalent lanthanide is fully encapsulated, the lanthanide ions are not, or at any rate less, in a position to interact with water or other OH-containing impurities. Moreover, it appears that any water present when such a complex is formed is stripped off the lanthanide. A further advantage of such a lanthanide complex is that it dissolves or mixes with the polymeric material far more readily than lanthanide salts or lanthanide oxides do. This is because the host molecule comprises organic material, just as the polymeric waveguides do. Hence, it is possible for the lanthanide to be incorporated into a polymeric optical fiber in a permanently anhydrous state in the form of a complex, thus maintaining the advantages of polymeric optical fibers over glass fibers. In this way a lanthanide-doped polymeric optical waveguide is provided which has all the above-mentioned advantages of polymeric optical waveguides to boot.
An organic ligand is used as a sensitizer. The organic ligand is excited in a strong absorption band to more efficiently populate the excited state of the rare-earth ion, after which the energy is transferred to an excited state of the rare-earth ion. A significant gain in excitation efficiency can be obtained via this route. The absorption coefficients of the (forbidden transitions of the) rare-earth ions are extremely low, typically 1-10 l molxe2x88x921 cmxe2x88x921, whereas organic ligands may have absorption coefficients which are 3-4 orders of magnitude higher, because allowed xcfx80xe2x80x94xcfx80* transitions are probed.
Preferred organic sensitizers are selected from fluorescein derivatives such as fluorexon, eosin, erythrosin, fluorescein, rose bengal, calcium green, and oregon green; triphenylmethane derivatives such as methylthymol blue, xylenol orange, brilliant blue, methyl green, and malachite green; porphyrin derivatives; rhodamine derivatives such as rhodamine 6G, tetrabromorhodamine, and lissamine; phenothiazine derivatives such as thionin and methylene blue; phenoxazine derivatives such as nile blue; coumarin derivatives; acridin derivatives such as acridin orange; (thio)indigo derivatives; carbocyanine derivatives; squaraine derivatives; buckminster fullerenes, and (na)phthalocyanine derivatives.
These compounds and derivatives are well-known to those skilled in the art. Coumarin derivatives, for instance, include 2- and 4-coumarins such as coumarin 120, 124, 445, 450, 490, 500, 503, and trifluoromethylcoumarin. Other sensitizers which absorb in the visible region can also be employed.
The state of the art of lanthanide complexes as active material in polymeric waveguides has been disclosed in EP 618,892. Important advantages of polymeric optical waveguides, as opposed to the conventionally applied inorganic optical waveguides, are the ease of handling the polymeric materials, which entails more flexibility and possibilities as to the devices which can be prepared (e.g., hybrids of switches and waveguides, direct coupling to laser source or detector), and reduction of manufacturing costs. Another advantage when an organic complex is used in a polymer host, is that higher doping of the waveguide can be realized than with inorganic ions in an inorganic host. This requires good compatibility of complex and host, which has been realized by the complexes described in EP 618,892, wherein a net neutral complex was obtained by application of a negatively charged ligand. A disadvantage of these organic complexes is that most organic ligands contain bonds which give highly energetic vibrational modes, which bonds usually are not present in inorganic hosts, or in the case of coordinated water molecules being present in the complex, can be removed by high-temperature treatment. Obviously, such treatment cannot be applied to organic complexes, which would decompose. It is therefore vital that the organic ligand have excellent shielding properties to keep away water and other Oxe2x80x94H and Nxe2x80x94H containing molecules from the rare-earth ion.
Closer evaluation of the photophysical properties of the complexes described in EP 618,892 shows that they are not very suitable for application in optical amplification. For instance, the complexes containing the near-IR luminescent rare-earth ion Er3+ exhibit rather short luminescence lifetimes (of the order of 1 xcexcs, which is 3 orders of magnitude below the lifetimes in inorganic matrices). Quenching effects are very pronounced for the near-IR emitting rare-earth ions, since they have excited states which are separated from the ground state by a slight energetic distance, so that they can be easily depopulated. Sensitized emission of the rare-earth ion with these applied complexes can only be achieved for excitation in the UV region (below 300 nm). Powers of several hundreds of mW are required to get optical amplification in such polymeric optical waveguides, which is not practical. Moreover, the use of excitation in the UV region is undesired in view of the inherent reactivity of the highly energetic photons and the significant propagation losses encountered due to the enhanced scatter efficiency and increased absorption values for the short wavelength light.
The advantage of the present invention is that the problems noted above have been overcome by the introduction into the complex of a sensitizer which absorbs in the visible region and is able to efficiently transfer its energy to the near-IR luminescent rare-earth ion. The present approach compensates for the reduction of the lifetime of the luminescent state, since the excitation efficiency is enlarged by 3 or 4 orders of magnitude. This results in a polymer-based fiber waveguide with the same or a better performance than that of the inorganic amplifiers, but with all the advantages of using polymer-based components. Such waveguides can be advantageously applied in optical devices, preferably those having a transversal pump configuration.
A survey of near-IR luminescence of the rare-earth metal ions of the invention is presented in Tables 1 and 2. Luminescence lifetimes (xcfx84lum) have been determined with a germanium detector (liquid nitrogen cooled) for lifetimes longer than 0.25 xcexcs, or with a near-IR sensitive photomultiplier for the shorter lifetimes. Table 1 demonstrates the advantageous properties of neodymium, ytterbium, and erbium with respect to the other lanthanide metals.
Of the rare-earth ions which have been examined, neodymium and erbium are the most suitable ones for application in optical waveguides, since they show emission in the wavelength regions applied in optical tele communication and data communication, i.e. around 1300 nm for Nd3+ and 1530 nm for Er3+. Nd3+ can also be applied for amplification in the 880 nm and 1060 nm range. Yb3+ is very suitable for amplification in the 980 nm region.
Compounds which are suitable for application as near-IR luminescent-active material in polymeric optical waveguides comprise several characteristic building blocks, i.e.,
a) a near-IR luminescent rare-earth ion, e.g., erbium, ytterbium, or neodymium ion, or a combination thereof,
b) a sensitizing moiety which is efficiently excited and able to transfer energy to the rare-earth ion. The sensitizing moiety should be as close to the ion as possible to make the energy transfer process more efficient.
c) a complexing agent (chelator or ligand) which shields the ion from quenching by solvent molecules, in particular from water which contains an Oxe2x80x94H moiety with particular quenching ability, and provides strong binding to the ion. It is desirable to provide the complexing agent with a site which can act as the sensitizer.
d) optionally, a functional group which covalently attaches the complex to a polymer, e.g. via an isothiocyanate, activated ester (such as N-hydroxysuccinimidyl ester), epoxide, or maleimide for coupling to amino groups; an amino group for coupling to epoxides; a maleimide or a halogenoacetamide for coupling to thiols; a hydroxy for reaction with acid chloride, isocyanate, or chloroformate; or a hydrazide for coupling to aldehydes, all of which methods are well-known to the artisan. Covalent attachment is optional, because the compound can be suitably applied also when it is present in the polymer as solid solution.
The near-IR luminescent complexes enable the design of low-cost excitation sources: thus visible light which can be obtained via inexpensive light sources, such as solid-state lasers, light emitting diodes, or conventional lamps, can be used for excitation.
The combination of organic ligands and rare-earth ions results in an improvement of the photochemical stability of the compounds. The triplet state of the sensitizer is efficiently quenched by the rare-earth ion, so that the effective lifetime of this state is significantly reduced, and no or at least less reactive singlet oxygen is formed.
To demonstrate the applicability of the proposed approach two sets of complexes have been investigated, i.e.:
a) complexes with a rare-earth ion in direct interaction with a sensitizer, such as fluorexon, which is a fluorescein derivative with a number of complexing groups; and
b) complexes with a rare-earth ion in interaction with a structural moiety of an organic molecule (e.g., a calix[4]arene) which provides sufficiently strong complexation behavior, sufficient coordinative groups to reduce the number of water molecules in the immediate vicinity of the rare-earth ion, and a sensitizer, preferably with coordinative groups attached to it. Preferably, the sensitizer contains a moiety which may provide interaction with the rare-earth ion (e.g., a diaza- or carboxylic acid moiety).
Examples of the direct interaction type, i.e. sensitizers with complexing ability, are fluorescein and derivatives thereof (e.g., fluorexone, eosin, erythrosin, fluorescein, Rose Bengal or tetrachloro-tetraiodofluorescein, Calcium Green, and Oregon Green), triphenylmethane derivatives (e.g., Methylthymol Blue, Xylenol Orange, Malachite Green, and Brilliant Blue), and porphyrins/texaphyrins, or (na)phthalocyanins.
Examples of the indirect interaction type are calix[4]arenes, xcex2-diketonates, and polyaminocarboxylic acids, such as EDTA, DTPA, and TRHA, all basically containing aminocarboxylate groups as complexing agents. Other complexing agents are, e.g., pyridinecarboxylic acid derivatives, orxe2x80x94in generalxe2x80x94any type of compound containing oxygen, nitrogen, phosphorous, or sulfur moieties which have complexing ability towards rare-earth metal ions.
Other examples of suitable sensitizers are rhodamine derivatives (e.g., Rhodamine 6G, tetrabromorhodamine, and Lissamine), thionin derivatives (e.g., thionin and Methylene Blue), acridin derivatives (e.g., Acridin orange), Nile Blue, Methyl Green, benzophenone derivatives (e.g., Michler""s Ketone and bisdimethylaminothiobenzophenone), and (thio)indigo. Also suitable are C60, C70, and C82 buckminster fullerenes and endofullerenes.
When optical lanthanide-doped waveguides are co-doped with Yb3+, the ytterbium ion is excited. The excited Yb3+ ion can transfer its energy to another lanthanide ion, which will be excited in turn.
The invention is further illustrated by the following examples.